The Diels-Alder Reaction is an organic chemical reaction between a conjugated diene and a substituted alkene (commonly termed the dienophile) to form a substituted cyclohexene system. The reaction can proceed even if some of the atoms in the newly-formed ring are not carbon. Some of the Diels-Alder reactions are reversible; the decomposition reaction of the cyclic system is then called the Retro-Diels-Alder.
This reaction belongs to a larger class known as 2 + 4 cycloadditions, which are characterized by the formation of a ring by a process involving two pi electrons of one reactant and four pi electrons of the other. Such reactions tend to be especially rapid because the transition state (see activation energy) involves six pi electrons delocalized around a ring, much like benzene. It therefore has some aromatic character and is particularly stable. By comparison, 2 + 2 and 4 + 4 cycloadditions tend to be much slower.
Otto Paul Hermann Diels and Kurt Alder were awarded the Nobel Prize in Chemistry in 1950 for their work on this reaction.
